Gas sensors are used for measuring the concentration of combustible gases and they include a catalyzer which is heated to a specific temperature such as 500.degree. C. In this way, the combustible gases are catalytically burned at the sensor surface while consuming a part of the oxygen contained in the gas to be measured.
As a measuring arrangement, it is conventional to utilize an active sensor (detector) and a passive sensor (compensator) in one or two separate measuring bridges. The detector reacts with the gas to be measured; whereas, the compensator functions essentially only for compensating for ambient influences. The compensator is heated to the same temperature as the detector. The measurement voltage below the lower explosion limit is proportional to the concentration of the combustible gas. This measurement voltage is equal to the difference between the detector voltage and the compensator voltage.
A measurement method for measuring the quantity of the components present in a gas and the measuring circuit associated therewith is disclosed in published European patent application 0 018 221 (U.S. patent application Ser. No. 031,750 and filed on Apr. 20, 1979). In the known measuring method, the detector element and the compensator element are arranged in separate measuring bridges and are connected to a common voltage supply. The supply currents are controlled in such a manner that the detector element as well as the compensator element are maintained at preselected temperatures. For this purpose, each bridge is provided with a switching unit in the form of a two-level controller in the voltage supply. This switching unit interrupts the supply current for a time duration when the preselected temperature is exceeded. The calibration of the detector element and compensator element takes place in such a manner that the output voltages are balanced to the same value for a reference gas which is free of combustible components.
For specific applications, the detector element and compensator element operate with voltages of unlike magnitude and therefore at unlike temperatures. The different output voltages are then balanced with a voltage adjusting arrangement at the summing point of the two measuring bridges so that the amplifiers connected downstream thereof see a balanced measuring bridge. This balance takes place again with a reference gas which is free of combustible components.
The known measurement method has the disadvantage that the detector element and compensator element can only operate at fixed predetermined temperatures and the power consumption (especially in the operational-ready phase where no combustible gas is present) is just as great as in the measuring phase with combustible gases. Especially in the case of portable apparatus, large batteries must be taken along to cover the current requirements in order that the power requirement of the measuring apparatus be covered. This makes these apparatus heavy and inconvenient to use.
U.S. Pat. No. 4,538,448 discloses a method for determining the explosion limit of a gas mixture. Here, a detector element is connected to a voltage supply and the supply current is controlled to a constant value after the switch-in time point. The voltage variation at the detector element in dependence upon time is registered by a measuring unit. An extrapolation function is computed from the voltage variation via support values and this extrapolation function is used to determine the explosion limit of the gas mixture.
In the known arrangement, it is disadvantageous that the detector element and also the compensator element are always connected to the operating current during the measuring phase and therefore continuously require the full power input. Furthermore, the system is not ready for measurement during pauses in measurement wherein the detector element is not connected to the supply current.